FIGS. 1A and 1B are schematic views of a conventional industrial method of fabricating smart cards of the aforementioned type. First of all, a connector 2 and a card body 12 are fabricated. Card body 12 has a cavity 14 for receiving connector 2. The connector includes external contact pads 4 arranged on an external face of support 6 and internal contact pads 8 arranged on the internal face of the support. External pads 4 are electrically connected to internal pads 8 by means known to those skilled in the art. Card body 12 includes a plurality of contact pads 16 intended to be connected to pads 8. Pads 16 are visible on a horizontal surface (parallel to the general plane of the card) of cavity 14. Each contact pad 16 is formed of a braze or solder 18, notably made of tin, deposited on an internal pad 20 of card body 12. Pads 20 are arranged at the surface of a support 22 associated with an electronic unit and/or an antenna incorporated in card body 12 and electrically connected to pads 20 via an electric circuit.
A film of conductive adhesive 10 and connector 2 are placed in cavity 14 of card body 12 and adhesive film 10 is arranged between the bottom of cavity 14 and connector 2. The connector is secured to card body 12 using a hot press device 26. FIG. 2 is a partial view of a card 28 obtained by the method of the prior art described here. According to this conventional method, conductive adhesive 10 forms a layer between internal contact pads 8 and contact pads 16 located on the horizontal surface defining the bottom of cavity 14.
Various tests carried out on cards of the type shown in FIG. 2 and an analysis of defective cards returned by various users show that the electrical connections between external connector 2 and contact pads 16 visible in cavity 14 are unreliable. Electronic cards, notably bank cards, must be able to undergo various mechanical stresses given that users generally carry the cards either in their wallet or in a flexible card holder. The bending and twisting to which cards 28, and their pairs of contact pads bonded to each others are subjected may result in the contact pads peeling off locally or in gaps forming which then break the electrical connection. These electronic cards therefore have problems of longevity.
A smart card incorporating an antenna is known from DE Patent No. 197 32 645. This card comprises a recess in which there appear two first contact pads, respectively electrically connected to the two ends of the antenna, and an electronic module inserted into the recess. The module has two second contact pads electrically connected to the first two pads. Each first pad is defined by the top surface of a truncated metal bump. In order to establish the electrical connection between the first and second pads, there is deposited on the first or second pads either a conductive adhesive, or a solder or braze for soldering or brazing the two pads to each other. The connections obtained via the second alternative are more robust and electrically improved. However, soldering requires a relatively large heat supply to reach the soldering temperature. This document proposes supplying heat through the electronic module support, which is generally formed of an insulating material which is a poor thermal conductor, for example a reinforced resin or plastic. A large supply of heat to the support is liable to deform the support and even damage the electronic module.
A smart card of the aforementioned type is known from WO Patent No 97/34247 wherein the solder material provided between the contact pads is incorporated in an adhesive film. The solder material is incorporated, in particular, in apertures made in the adhesive film, which is then placed against the electronic module substrate so that the solder is superposed on the internal contact pads of the module. Finally, the adhesive is activated and the solder material is melted by supplying heat through the insulating support of the electronic module. Thus, there is the same problem as in the preceding document. Moreover, filling the apertures of an adhesive film with solder material before the film is assembled to the electronic module causes fabrication problems, since it is not easy to ensure that the solder material remains in the apertures in the adhesive film until it is assembled to the electronic module. Thus, a variant provides for the introduction of conductive particles into an adhesive film in the areas provided for the solders.
This document WO 97/34247 also proposes a particular embodiment shown in FIG. 2. In this case, a module is provided with first external contact pads, which are electrically connected to second contact pads, flush with a horizontal surface of the recess made in the card body, via solder filled channels which are formed through the insulating support of the electronic module and through the first external contact pads. This embodiment causes several problems. First of all, there is no guarantee that the channels will be filled with solder once the solder melts. In particular, if the solder introduced into the channels is initially in paste form to facilitate insertion into the channels, when it melts during assembly this causes a contraction of the solder and the material connection with the lateral surface of the holes in the external pads is not guaranteed, especially if the thickness of the external pads is relatively small. Next, the hole made in the external contact pad makes the card unattractive which is incompatible with a high quality card. This risk seems to be confirmed by the drawing in FIG. 2 of WO Patent No 97/34247 where the solder is below the level of the top surface of the external pads. There is therefore a real problem of reliability. Finally, these holes in the external pads cause a problem for card readers, which generally have pressure pads or feeler needles which are liable to be damaged when the card is inserted into or removed from the reader. Further, the melted and solidified solder (for example tin) is generally much softer than the metal forming the external pads (copper with a gold flash). Therefore, if the holes in the external pads are properly filled with solder, the pressure pads or needles of a card reader will spread the solder over the external pads when the card is inserted or removed. This has several negative consequences: First of all, the reader head is soiled with the solder. Secondly, if the quantity of solder spread is relatively large, a short circuit may even occur between two contact pads. Thirdly, the contact pads are also soiled with the spread solder; which is unattractive and unacceptable. In any event, it is clear that the diameter of the solder channels must necessarily be small so that the resulting holes in the external pads are as small as possible to limit the aforementioned problems. However, with small solder channels, it is difficult to provide the heat required to form a solder up to the second contact pads of the antenna.